minerals Article Removal of Pb from Water: The Effectiveness of Gypsum and Calcite Mixtures Ana Roza Llera 1 , Amalia Jimenez 1,* and Lurdes Fernández-Díaz 2,3 1 Department of Geology, University of Oviedo, 33005 Oviedo, Spain; [email protected] 2 Dpto. Cristalografía y Mineralogía, Universidad Complutense de Madrid, José Antonio Novais, 2, 28040 Madrid, Spain; [email protected] 3 Instituto de Geociencias (UCM, CSIC), Ciudad Universitaria, 28040 Madrid, Spain * Correspondence: [email protected] Abstract: Anthropogenic lead pollution is an environmental problem that threatens the quality of soils and waters and endangers living organisms in numerous surface and subsurface habitats. Lead coprecipitation on mineral surfaces through dissolution-recrystallization processes has long-term effects on lead bioavailability. Gypsum and calcite are among the most abundant and reactive rock forming minerals present in numerous geological settings. In this work, we studied the interaction of slightly acidic (pHi = 5.5) Pb-bearing aqueous solutions ([Pb]i = 1 and 10 mM) with crystals of gypsum and/or calcite under atmospheric conditions. This interaction resulted in a reduction of the concentration of lead in the liquid phase due to the precipitation of newly formed Pb-bearing solid phases. The extent of this Pb removal mainly depended on the nature of the primary mineral phase involved in the interaction. Thus, when gypsum was the only solid phase initially present in the system, the Pb-bearing liquid-gypsum interaction resulted in Pb removals in the 98–99.8% range, regardless of [Pb]i. In contrast, when the interaction took place with calcite, Pb removal strongly depended on [Pb]i. It reached 99% when [Pb]i = 1 mM, while it was much more modest (~13%) when [Pb]i = 10 mM. Interestingly, Pb-removal was maximized for both [Pb]i (99.9% for solutions with [Pb] = 10 mM and 99.7% for solutions with [Pb] = 1 mM) when Pb-polluted solutions i i simultaneously interacted with gypsum and calcite crystals. Despite the large Pb removals found in Citation: Roza Llera, A.; Jimenez, A.; most of the cases studied, the final Pb concentration ([Pb]f) in the liquid phase was always well above Fernández-Díaz, L. Removal of Pb the maximum permitted in drinking water (0.01 ppm), with the minimum ([Pb]f = 0.7 ppm) being from Water: The Effectiveness of obtained for solutions with [Pb]i = 1 mM after their interaction with mixtures of gypsum and calcite Gypsum and Calcite Mixtures. crystals. This result suggests that integrating the use of mixtures of gypsum-calcite crystals might Minerals 2021, 11, 66. https:// help to develop more efficient strategies for in-situ decontaminating Pb-polluted waters through doi.org/ 10.3390/min11010066 mineral coprecipitation processes. Received: 11 November 2020 Keywords: lead; removal; calcite; gypsum; coprecipitation Accepted: 31 December 2020 Published: 11 January 2021 Publisher’s Note: MDPI stays neu- tral with regard to jurisdictional clai- 1. Introduction ms in published maps and institutio- The quantity and the quality of available freshwater have been steadily declining nal affiliations. over the last century in both industrialized and developing countries. Human activities like farming, mining, or manufacturing, which provoke the continuous rise of salinity and pollutant contents of freshwater in many sites worldwide overworld, are the main factors of this decline [1–4], as well as the adoption of new lifestyles by large groups of Copyright: © 2021 by the authors. Li- censee MDPI, Basel, Switzerland. citizens in densely populated cities, which is responsible for the general increase of the drug This article is an open access article and pharmaceutic contents in wastewaters. The current scenario of progressive climate distributed under the terms and con- warming, which sketches a future of groundwaters with increasing salinity, adds stress on ditions of the Creative Commons At- the availability of clean fresh water supplies around the world [5]. tribution (CC BY) license (https:// Pb is a most hazardous heavy metal whose concentration in the atmosphere, soils, and creativecommons.org/licenses/by/ waters underwent a striking worldwide increase during the 20th century associated with 4.0/). the development of the automobile industry and the generalized use of leaded gasolines Minerals 2021, 11, 66. https://doi.org/10.3390/min11010066 https://www.mdpi.com/journal/minerals Minerals 2021, 11, 66 2 of 21 until their banning in the 1980s [6,7]. Other industrial activities like the production of ammunition, batteries, pigments, and certain glasses have also contributed anthropogenic Pb inputs to the hydrosphere and the biosphere [8–12]. Mining activities have contributed to the long-term release of Pb as a result of the interaction of large volumes of fine-grained Pb-ores exposed in tails with groundwaters and running waters [13–17]. Furthermore, most catastrophic emissions of Pb environments, which have caused lasting damage in sub- aquatically and subaerially with great impact on soil fertility and productivity and wildlife diversity, have resulted from punctual wastewaters spills in mine sites and tails [18–20]. Pb was the main component of plumbing instalments in worldwide households until the 1970s, after which the use of lead pipes rapidly declined in industrialized countries. Interestingly, Pb-related pollution of drinking water has become a hot topic in recent years as information on the relationship between the presence of lead-tainted water pipes in old buildings and high Pb contents being found in the blood and bones of children in deprived neighborhoods in industrialized countries has reached the news [12,21]. Since Pb is potentially toxic to mammals, water pollution that results in excessive Pb intake often has dramatic impacts on human health, including long-lasting damage to the nervous, skeletal, circulatory, enzymatic, endocrine, and immune systems [17,18,22–26]. This damage is particularly dramatic in the case of sensitive populations, like children, pregnant women, and elderly people [27–30]. The bioavailability of dissolved Pb in natural environments is largely controlled by sorption processes that develop during the interaction between Pb-polluted waters and the surface of rock-forming minerals like calcite and gypsum, which are especially abundant in sedimentary basins as main constituents of limestones (calcite) and sulfate evaporitic de- posits (gypsum) [31–33]. Sorption of pollutants by mineral surfaces can take place through different mechanisms, including adsorption, absorption, and surface precipitation [34–37]. The first mechanism, adsorption, involves the bonding of the pollutant to the mineral surface by chemical bonds that can be relatively strong, or through longer-range coulomb forces and/or hydrogen bonds. The second mechanism, absorption, requires the diffusion of the pollutant into the structure of the mineral. Adsorption contribution to sorption is sig- nificant in the case of minerals like zeolites, whose structures contain large, interconnected channels that allow for ion exchange to occur at relevant rates [38]. In contrast, under the temperature conditions reigning in surface and subsurface environments, the kinetics ion diffusion through carbonate and sulfate mineral structures is most sluggish and absorption contribution to sorption can be considered negligible. The third mechanism, surface pre- cipitation, involves the co-precipitation of the pollutant which incorporates in the crystal structure of new mineral phase. This new phase forms through the chemical reaction between the pollutant and ions that are released to the fluid phase due to the dissolution of the primary mineral. This sorption mechanism commonly progresses by the develop- ment of coupled dissolution-crystallization reactions. Coupled dissolution-crystallization reactions are commonplace in surface and subsurface geological environments, where they affect primary mineral phases that can be highly soluble, like halite (NaCl), relatively soluble, like gypsum, or sparingly soluble [39–42], like leucite or wollastonite [43–45]. Further evidence of the widespread nature of these reactions in natural environments have also been provided by a variety of macroscopic and microscopic experiments [46,47]. In fact, coupled dissolution reactions that involve different sulfate and carbonate minerals are considered an effective long-term way of removing inorganic pollutants from natural water and wastewaters [48–51] as well as reducing the increase atmospheric CO2 [52,53]. Indeed, in situ AFM imaging has unraveled that coupled dissolution–crystallization reac- tions that involve calcite effectively remove pollutants like manganese [54], cadmium [37], lead [55], phosphate [56,57], and chromate [58] from aqueous solutions. Similarly, coupled dissolution-crystallization reactions that occur during gypsum-aqueous solution interac- tion can result in the sequestration of dissolved components like barium and strontium [59], lead [60], arsenate [61], phosphate [62,63], and carbonate [64] through their immobilization in the structure of new phase. Minerals 2021, 11, 66 3 of 21 Different authors have applied microscopic and macroscopic approaches to study the influence of the interaction of Pb-bearing fluids and common calcium-bearing carbonate and sulfate rock-forming minerals (namely, calcite, aragonite, gypsum, and anhydrite) in the fate of dissolved Pb in natural environments. This interaction has been addressed in numerous studies, most of